Lab Simulation 14 Guidelines This is a lab item in which tasks will be performed on virtual devices. - Refer to the Tasks tab to view the tasks for this lab item. - Refer to the Topology tab to access

OSPF DR/BDR Election Control - Lab Explanation --- Overall Goal OSPF elects a Designated Router (DR) and Backup Designated Router (BDR) on multi-access networks (like Ethernet) to reduce OSPF traffic - instead of every router forming adjacencies with every other router, all route

Submitted by lucia.co· Mar 6, 2026Infrastructure

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Lab Simulation 14 Guidelines This is a lab item in which tasks will be performed on virtual devices. - Refer to the Tasks tab to view the tasks for this lab item. - Refer to the Topology tab to access the device console(s) and perform the tasks. - Console access is available for all required devices by clicking the device icon or using the tab(s) above the console window. - All necessary preconfigurations have been applied. - Do not change the enable password or hostname for any device. - Save your configurations to NVRAM before moving to the next item. - Click Next at the bottom of the screen to submit this lab and move to the next question. - When Next is clicked, the lab closes and cannot be reopened. Topology Tasks OSPF is partially configured. Complete the OSPF configurations to achieve these goals: 1. Configure R3 and R20 so they do not participate in a DR/BDR election process in Area 20. 2. Configure R10 so it is always the DR for Area 10. Do not change the router ID. Answer: Task 1. Configure R3 and R20 so they do not participate in a DR/BDR election process in Area 20. Area 20 only has two routers so if we use "ip ospf priority 0" command on them (so that they do not participate in DR/BDR election process) then the OSPF process may fail as there is no DR/BDR to process OSPF packets. A better solution is to set OSPF network type to point-to- point. On R3: R3(config)#interface e0/1 R3(config-if)#ip ospf network point-to-point On R20: R20(config)#interface e0/0 R20(config-if)#ip ospf network point-to-point Make sure the election is reset on both routers: R3#,R20#clear ip ospf process Reset all OSPF processes? [no]: y Verification On both R3 and R20 use the "show ip ospf neighbor" command, we will see both neighbors show the "DROTHER" state. For example on R3: R3#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface ... 10.2.203.20 0 FULL/- 00:00:10 10.2.203.20 Ethernet0/1 Note: We should check the "Address" field (as the "Neighbor ID" field does not show what is the router clearly) to identify the neighbor. In the above output, from the "Address" field of 10.2.203.20, we can identify this is R20. Task 2. Configure R10 so it is always the DR for Area 10. Do not change the router ID. We can configure R10 to be the DR with highest OSPF priority value: R10(config)#interface e0/0 R10(config-if)#ip ospf priority 255 Make sure the election is reset on R2 & R10: R2#,R10#clear ip ospf process Reset all OSPF processes? [no]: y Verification On R2 use the "show ip ospf neighbor" command, we will see R10 (with "Address" field of 10.1.102.10) shows the "DR" state. Save the configuration We changed the configuration on R3, R10 and R20 so we need to save the configuration on all of them: R3#,R10#, R20# copy running-config startup-config

Exhibits

Explanation

OSPF DR/BDR Election Control - Lab Explanation

Overall Goal

OSPF elects a Designated Router (DR) and Backup Designated Router (BDR) on multi-access networks (like Ethernet) to reduce OSPF traffic - instead of every router forming adjacencies with every other router, all routers form adjacencies only with the DR/BDR. This lab controls who participates in that election (and who wins it).

Task 1: Prevent DR/BDR Election on Area 20 (R3 & R20)

Why point-to-point instead of priority 0?

Setting ip ospf priority 0 tells a router "never become DR or BDR." But if both routers in Area 20 have priority 0, no DR can be elected, and OSPF adjacency will never reach FULL state - the network breaks.

The correct fix is ip ospf network point-to-point, which tells OSPF: "treat this link as a direct two-router connection - no DR/BDR election needed at all." This is valid because DR/BDR only matters on multi-access segments with 3+ routers.

Step-by-step reasoning:

Step 1 & 2 - Apply point-to-point on R3 e0/1 and R20 e0/0 These are the interfaces connecting R3 and R20 in Area 20. Both sides must match - if only one side is point-to-point, the network types mismatch and OSPF adjacency drops entirely.

Step 3 - Clear OSPF processes on R3 and R20 OSPF network type changes don't take effect on existing adjacencies without a reset. clear ip ospf process forces OSPF to re-negotiate the relationship using the new network type. Skipping this means the old DR/BDR state persists until the dead timer expires (up to 40 seconds by default), and the exam grader may check state immediately.

Step 4 - Verify with show ip ospf neighbor The State column should show FULL/- (the - means no DR role assigned). The answer notes you should use the Address field, not the Neighbor ID, to positively identify R20 (router IDs can be confusing if they don't match interface IPs).

What breaks if skipped: Mismatched network types = no adjacency. Both set to priority 0 = no DR elected = OSPF stuck in EXSTART/EXCHANGE, never reaching FULL.

Task 2: Force R10 to Always Win the DR Election in Area 10

Why priority 255?

DR election is determined by highest OSPF priority (default: 1). Setting R10 to 255 (the maximum) guarantees it wins against any other router with a lower priority. The instruction says "do not change the router ID" - this rules out using a higher router ID as the tiebreaker.

Step 5 - Set ip ospf priority 255 on R10 e0/0 Only the interface connecting to Area 10 needs this change. Higher priority = DR. This is deterministic and survives reboots (unlike router ID tiebreakers which depend on discovery order).

Step 6 - Clear OSPF on R2 and R10 DR elections are not preemptive by default - an existing DR keeps its role until OSPF resets. Without clearing, R10's new priority won't be recognized until the next election. You must reset both routers in the segment so a fresh election occurs.

Step 7 - Verify on R2 with show ip ospf neighbor R10's entry (identifiable by Address 10.1.102.10) should show state FULL/DR. R2 itself will show as BDR or DROTHER in its own neighbor table.

What breaks if skipped: Without clearing, the old DR retains its role. The lab grader will see R10 is not the DR and mark the task wrong even though the config is correct.

Step 8 - Save Configurations (`copy run start`)

Changes in RAM are lost on reload. Must be done on R3, R10, and R20 (the three routers you changed). R2 was only used for clearing/verification - its config was not modified.

What breaks if skipped: The exam explicitly requires saving to NVRAM. The grader may reload devices to verify persistence.

Memory Tip

Goal	Tool	Why
No DR/BDR on a 2-router segment	`network point-to-point`	Eliminates election entirely - safe with 2 routers
Always win the DR election	`priority 255`	Highest value wins; max is 255
Never win (but keep adjacency)	`priority 0`	Only safe when other routers can still elect a DR

The golden rule: After any OSPF election-related change, always clear ip ospf process on all routers on that segment - OSPF elections are not preemptive.

Topics

#OSPF#DR/BDR Election#OSPF Network Type#OSPF Priority

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